HVDC power transmission is becoming increasingly important due to the steadily rising need for bulk power delivery and power grid interconnections. In particular, using HVDC transmission based on voltage source converters (VSC), power can be transmitted with underground cables and overhead lines which are embedded or overlapped with high voltage alternating current (AC) transmission networks. An attractive feature of HVDC transmission is that the direction of power transmission can be changed by changing the direction of current.
An HVDC grid comprises multiple VSCs, also referred to as terminals, which convert an AC power source for transmission over an HVDC transmission line, i.e., cables or overhead lines. Different configurations of HVDC transmission lines are known, such as monopole, symmetric monopole, and bipolar. For instance, a bipolar HVDC transmission line comprises a positive pole line, a negative pole line, and a metallic return line connected to ground. Within the grid, each terminal may be connected to multiple terminals by HVDC transmission lines resulting in different types of topologies. Such a multiple terminal grid enables efficient congestion management and has an improved stability against disturbances.
Direct current (DC) circuit breakers are commonly used for isolating faulty components, such as transmission lines, in HVDC grids. Due to the low inductance of DC transmission lines, as compared to AC systems, HVDC systems suffer from a high rate of rise of fault induced currents. Thus, the tripping of DC breakers has to be effected before the rising current exceeds the interrupting capacity of the breakers. In order to provide an improved stability of such grids, a breaker failure protection of DC circuit breakers is desirable.